Fix type error when extending Sandbox class (#264)

* Make getSandbox and SandboxEnv generic for subclasses

Fixes type error when extending Sandbox class.
DurableObjectNamespace<T> is invariant, so getSandbox
needed to be generic to accept subclass namespaces.

* Add changeset

Author: ghostwriternr

.changeset/generic-sandbox-types.md

@@ -0,0 +1,5 @@
+---
+'@cloudflare/sandbox': patch
+---
+
+Fix type error when extending Sandbox class by making getSandbox and SandboxEnv generic

packages/sandbox/src/request-handler.ts

@@ -3,8 +3,8 @@ import { createLogger, type LogContext, TraceContext } from '@repo/shared';
 import { getSandbox, type Sandbox } from './sandbox';
 import { sanitizeSandboxId, validatePort } from './security';
 
-export interface SandboxEnv {
-  Sandbox: DurableObjectNamespace<Sandbox>;
+export interface SandboxEnv<T extends Sandbox<any> = Sandbox<any>> {
+  Sandbox: DurableObjectNamespace<T>;
 }
 
 export interface RouteInfo {
@@ -14,10 +14,10 @@ export interface RouteInfo {
   token: string;
 }
 
-export async function proxyToSandbox<E extends SandboxEnv>(
-  request: Request,
-  env: E
-): Promise<Response | null> {
+export async function proxyToSandbox<
+  T extends Sandbox<any>,
+  E extends SandboxEnv<T>
+>(request: Request, env: E): Promise<Response | null> {
   // Create logger context for this request
   const traceId =
     TraceContext.fromHeaders(request.headers) || TraceContext.generate();

packages/sandbox/src/sandbox.ts

@@ -45,11 +45,11 @@ import {
 import type { MountInfo } from './storage-mount/types';
 import { SDK_VERSION } from './version';
 
-export function getSandbox(
-  ns: DurableObjectNamespace<Sandbox>,
+export function getSandbox<T extends Sandbox<any>>(
+  ns: DurableObjectNamespace<T>,
   id: string,
   options?: SandboxOptions
-): Sandbox {
+): T {
   const sanitizedId = sanitizeSandboxId(id);
   const effectiveId = options?.normalizeId
     ? sanitizedId.toLowerCase()
@@ -65,7 +65,7 @@ export function getSandbox(
     );
   }
 
-  const stub = getContainer(ns, effectiveId) as unknown as Sandbox;
+  const stub = getContainer(ns, effectiveId);
 
   stub.setSandboxName?.(effectiveId, options?.normalizeId);
 
@@ -87,7 +87,7 @@ export function getSandbox(
 
   return Object.assign(stub, {
     wsConnect: connect(stub)
-  });
+  }) as T;
 }
 
 export function connect(stub: {